Process and apparatus for selective condensation of metals



J1me 1954 H. s. CALDWELL, JR., ETAL 3,136,627

PROCESS AND APPARATUS FOR SELECTIVE CONDENSATION OF METALS Filed June11, 1959 2 Sheets-Sheet l I WEIGHT or CO/VDENSATE r- WEIGHT 0FCONDENSATE TEMP COOLA/VT FLU/D 1 OUT t TO VACUUM INVENTORS. HERBERT SC4LDWELL JR MAX J SPENDLOl/E ATTORNEY CRUC/BLE June 9, 1964 H. s.CALDWELL, JR., ETAL 3,136,527

PROCESS AND APPARATUS FOR SELECTIVE CONDENSATION OF METALS 2Sheets-Sheet 2 Filed June 11, 1959 70 VA C UUM COOL/N6 FLU/D COOL/N6FLU/D INVENTORS. HERBERT s CALDWELL,JR

MAX J SPENDLOI/E %MA% ATTORNEY United States Patent 3,136,627 PROCESSAND APPARATUS FGR SELECTIVE v QGNDENSATION 0F METALS Herbert S.Caldwell, Jr., Hyattsville, and Max J. Spendlove, Takoma Park, Md.,assignors to the United States of America as represented by theSecretary of the Interior Filed June 11, 1959, Ser. No. 819,737 Claims.(Cl. 75-63) (Granted under Title 35, US. Code (1952), see. 266) Theinvention herein described and claimed may be manufactured and used byor for the Government of the United States of America for governmentalpurposes without the payment of royalties therein or therefor.

This invention relates to the selective separation of components from amixture. More specifically, it relates to the separation of componentsof such a mixture by distillation and selective condensation of theconstituent compounds or elements in a high state of purity. In the mostspecific form it pertains to the separation of individual metals in ahigh state of purity from a mixture of metals or an alloy.

When distilling a charge of a military incendiary alloy, at 600 0,assuming that the partial pressures obey Raoults law, the vapor can beshown to be initially nearly all cadmium with only a small percentmagnesium and a fraction of one percent of zinc, and no aluminum.However, as the distillation progresses, the vapor composition changesso that it can be shown by calculation from the Rayleigh distillationequation that the vapor contains equal proportions of magnesium andcadmium after about 10 percent of the alloy is evaporated and that whenthe alloy is 30 percent evaporated the vapor contains roughly about 75percent Mg and 25 percent Cd. Cadmium is known to have a negativedeviation from Raoults law, thus indicating that cadmium cannot beseparated from magnesium by simple distillation.

This was borne out in actual tests wherein a binary alloy of 80 Mg- Cdwas placed in a crucible having a condenser connected thereto as shownin FIG. 1, de scribed below. Runs were made under vacuum at temperaturesof 550C. and 700 C. and condensate was removed from the wall of thecondenser at intervals. The following table gives the composition, andthe percent of evaporated charge.

Table 1.Simple Distillation Tests on Prepared 80-20 Magnesium-CadmiumAlloys at 550 and 700 C.

These results indicate that separation by a fractional distillationmethod is not feasible.

It is interesting to note that, if the partial pressures followedRaoults law, vapor having a composition of 80 Avei-age composition ofincendiary alloy:

Percent by? wt.

Elements Cd 19.1 A1 4.13 Z11 .45

Other .74:

3,136,627 Patented June 9, 1964:

Mg-2O Cd would have to come from an alloy containing 99 percent Mg andonly 1 percent Cd. This indicates that the composition of the surface isenriched in magnesium because of the relative slowness with whichcadmium diffuses from the interior of the melt to re place that whichhas been evaporated. Cadmium, having an atomic weight much greater thanmagnesium, will not diffuse to the surface as rapidly as it is initiallyevaporated. Furthermore, as the surface is no longer enriched in cadmiumthe liquidus temperature rises so at any temperature below the meltingpoint of magnesium the surface may be covered with a film of solidmagnesium-rich alloy, which also inhibits the evaporation of cadmium.

Briefly, this invention consists of a method and apparatus separatingmixtures, which are diflicult to separate by fractional distillationbecause of nearly equal partial vapor pressures, by selectivecondensation. The apparatus consists of a retort having a lower crucibleportion and an upper cooled condenser portion. The mixture is placed inthe crucible, heated under reduced pressure and the evolved vapors passfrom the crucible to the condenser along a tortuous path provided by abafile within the lower portion of the condenser.

less volatile constituents condense near the entrance to the baffle. Thevapors impinge on a plate cooled to condense some of the less volatileconstituents admixed with some of the more volatile components, and theuncondensed vapors are reheated by heat exchange within the baflle.After leaving the baflle the more volatile constituents condense on thecondenser walls.

It is an object of this invention to provide an improved method andapparatus for separating the components of a mixture of substances whosepartial vapor pressures are nearly equal to reasonable distillationtemperatures, by selective condensation under reduced pressure.

It is a further object of this invention to provide an apparatusforseparation by selective condensation, consisting of a retort having acrucible and a connecting condenser, wherein the mixed material isheated under reduced pressure in the crucible and the evolved vaporspass from the hot crucible region to the cool condenser, and whereinbaffie means are provided within the condenser for reversing the vaporpath and causing a reheating of the vapors whereby a more clearcutseparation of the various constituents in condensation is obtained. Saidbaffie means provide tortuous path for the vapors and act by virtue ofthe restrictive effect, to increase the pressure in the crucible portionof the retort, whereby the release of the vapors from the bafllepassages into the rest of the condenser area results in an expansion andresultant cooling of said vapors.

The various figures in the drawing are as follows:

FIG. 1 shows, diagrammatically, a simple retort having a lower crucibleportion and an upper condenser portion.

FIGS. 2 and 3 show a group of graphs wherein the ordinate in each caseis the distance along the distillation zone of FIG. 2 and the abscissasare as indicated.

FIG. 4 shows a View in cross section of one embodiment of the retort.FIG. 5 is a fragmentary view of a portion of FIG. 1 without thecondensed metal.

Without being bound to any theory, the failure of separation byfractional distillation can be explained as follows:

Consider a simple binary vapor composed of constituents A and B whosepartial vapor pressures are nearly equal at the temperature andcomposition range considered, A being the more volatile component.Although there is a degree of selectivity in condensing theconstituents, the zones in which A and B condense overlap. Thecomposition of the condensate at any point depends on the temperature,the relative concentrations of the constituents in the vapor, and therelative rates of diffusion of the two species of vapor molecules. Theseparation is not sharp because as the B molecules they leave apreponderance of A molecules in the vicinity near the condenser wall.The large number of A molecules near the condenser wall increases theprobability that some of them will condense prematurely with the Bmolecules. Under the circumstances, some of B molecules diffuse muchfarther than they would otherwise, and many of them condense belatedlywith the A species.

This is illustrtaed in FIG. 2, portion (a) showing the temperaturedistribution along the length of the vapor path through the crucible andcondenser of FIG. 1. FIG- URE 2(b) shows the relative distribution ofthe constituents Within the condenser. It is evident that constitutent Bis more abundant at the entrance zones of the condenser, whileconstituent A, being more volatile is condensed in greatest abundance ata more remote location in the condenser. However, the B condensatecontains an appreciable amount of A and the A condensate contains anappreciable amount of B. V In other words, the respective constituentsare not selectively condensed so that they can be separated into tworelatively pure fractions.

By increasing the temperature of the vapors at that location in theirpath after the bulk of element B has condensed, the separation of A fromB may be enhanced. This is shown in FIG. 3 where the graphs show theeifects of heating this vapor.

We have found the best mode of heating the vapor is to insert a bathe ofsuitable design in the retort, so as to re erse vapor flow for heatexchange relationship within the condenser. This is best shown in FIG.4.

The apparatus, generally denoted as retort, consists of a cruciblemember 1 having mounted atthe top in any suitable manner a plate 2having a concentric opening 3 therein. A tube 4 mounted over the opening3 extends vertically upward. Covering the tube 4 is 'a tubular cap 5having apertures 6, at the bottom, and a cover 7 at the top.

Fastened to the upper part of the crucible by any suit- Because thebaffle reduces the diameter of the vapor path, the pressure buildup inthe crucible 1 is greater than would normally exist without said bathe.Near the junction of plate 2 and crucible 1, there is a sharp drop intemperature due to the removal of heat by conduction through the outerwall. Condensation of at least some of the less volatile componentsoccur at location 15 as solid deposits 18. The more volatile componentsremain in vapor state along with some of the less volatile constituents,and pass through tube 4. Only slight cooling occurs during the passageof the vapors through 4, and the heat that is lost is absorbed by thewalls of tube 4. In the downward passage of the vapors between 4 and 5,most of the heat lost in transit is reabsorbed from the outer surface oftube 4. Some heat is lost from vapors condensing on cover 7, which ismuch cooler than the temperature of the impinging vapors because of itsproximity to water cooling jacket 10. By pro-per spacing and control ofheat exchange conditions, cover '7 may be maintained at a temperaturebelow the condensing point of the less volatile constituents, and abovethe condensing point of the more volatile constituents. Solids 17condense on cap 7 as shown. The more volatile constituents remain in thevapor state and pass downwardly through the relatively restrictedannular space 4a between tube 4 and cap 5, during which the heat isabsorbed from the walls of 4. The pressure and temperature of the morevolatile constituent vapor is thus maintm'ned above the condensationconditions until it emerges from the annular space 5a between cap 5 andsleeve 8. At this point there is a sudden expansion of relativelycompressed vapors as they enter the large cool condenser space at 16.The resulting sharp drop in temperature causes the more volatileconstituents to condense immediately on the walls of the condenser asdeposit 19.

The operation of this invention is shown by the following specificexample.

A military incendiary alloy of the composition given in the footnote wasplaced in crucible 1 and heated by electrical induction coil 13. Vacuumwas applied through tube 12. The conditions'and results of two runs aresummarized in the following Table 2 as tests C and D.

Table 2.Segregati0n of Constitutents in Condensates Produced bySelective Condensation With a Vapor System Chemical analysis, percentPercent Test Temp, Time, pressure, Material Weight, of total No. 0. min.microns gm. conden- Mg Cd A1 Zn sate ghafigaug 72. 19.3g .4. 3 O. 5, 588on ensa e17" 8 17. .01 581 18.2 0 Condensate 19 0.1 98.4 .01 .87 47s14.s Condensate 18.. 98. 8 .1 .01 Trace 2,134 66. 9 ghalgitgenu1E 7;.$9.; 4. 3 .45 4, 920 on ensa c 17 6 5. .01 369 17.1 D 550 300 50Condensate 19- .3 99.4 .01 .30 303 14.3 Condensate '18 99. 9 Trace raceTrace 1, 478 68. 6

able manner is a sleeve member 8. In the embodiment shown, see FIG. 5,this is done by means of a shoulder at the upper end of crucible ll.-Plate 2 and sleeve 8 fit into the shoulder 9 in the manner shown, andthe structure can therefore be readily disassembled. Sleeve 8 is cooledby a heat exchanger it), attached around the outside of the sleeve tocover substantially all the outer surface of the sleeve between itsupper edge and that circumferential part of the sleeve in closeproximity to the cover 7 of the cap. .The upper part of the sleeve iscovered by a plate 11. Tube 12 passing through plate 11 is connected toa vacuum pump. A l joints shown are close fitting and/ or gasketed toprevent leakage.

In operation, the vapor passes through tube 4 and is cooled slightly.Heat is absorbed by the walls of 4 and is transferred to the vaporpassing outside of the tube, which produces the retrograde temperaturedistribution shown in FIG. 3.

As shown in the table, condensates 18 and 19 are relatively pure metal.Condensate 17 is a mixture of cadmium and magnesium which can berecycled in the process.

Analogous results are obtained by employing a zinc cadmium and azinc-magnesium alloy in the above method.

it is not necessary that in the case of metal mixtures that thecomponents be alloyed together. Under certain circumstances it isfeasible to separate particulate mixtures of metals in this manner.While in the example the alloy was heated to melting temperature, thisis not essential. The-process may be operated where feasible undersublimation conditions, Where the vapor pressures are high enough tocause evaporation at a relatively rapid rate. Materials other thanmetals may be separated in this manner, such as mixtures of organiccompounds or inorgnic salts.

The materials of construction employed in the retort are governed byproperties of the substances treated. In

the case of the alloy shown, carbon lined mild steel is suitable. Withorganic or inorganic compounds, glass may be necessary.

It is obvious various other arrangements and substitutions may be madein the method and apparatus without changing the esential inventiveconcept. Thus, instead of having the cooling means surrounding the upperzone of the condensing sleeve, a cooling well may be mounted in saidzone. Any of the common means known to the art may be employed forjoining the various sections of the retort.

We claim:

l. A method for separating a mixture of metals having different meltingpoints and volatilities by vacuum distillation and selectivecondensation which comprises; placing the mixture into an evaporationzone, heating the mixture to a temperature at which at least some of themetals have vapor pressures sufiicient to cause evaporation at arelatively rapid rate, passing the evolved vapors to a condensationzone, said vapor tracing a path having an initial, intermediate andfinal section, each of said sections having an entrance and a rearwardportion, said evaporating and condensation zones being under a reducedpressure, cooling the entrance of the initial section sutficiently tocondense at least one of the less volatile metals, passing the vaporsthrough the initial section of the vapor path with only slight cooling,impinging said vapors at the exit of said initial zone onto a surfacecooled to just below the condensing point of said less volatile metal,passing the uncondensed vapors through the intermediate section,increasing the temperature gradient along the vapor path of theintermediate section sufficiently for heating the vapors to prevent thecondensation of the more volatile metals, passing the vapors through thefinal path section, and decreasing the temperature gradient along thevapor path of said final section for cooling the vapors whereby at leastone of the more volatile substances is condensed.

2. A method for separating a mixture of metals having different meltingpoints and volatilities by vacuum distillation and selectivecondensation, which comprises; placing the mixture into an evaporationzone, heating the mixture to a temperature at which at least some of themetals have vapor pressures sufiicient to cause evaporation at arelatively rapid rate, passing the evolved vapors upwardly from saidevaportion zone, cooling the vapors sufficiently to condense at leastone of the less volatile metals, passing the remaining vapors through afirst restricted, elongated zone, impinging said vapors at the end ofsaid restricted zone onto a surface cooled to just below the condensingpoint of said less volatile metal, whereby an additional less volatilemetal condenses thereupon, passing said vapors downwardly through afirst annular space surrounding said elongated zone and in heat exchangerelationship therewith, whereby the vapors in the first annular spaceare heated, passing the vapors from said first annular space upwardlyinto a second annular space surrounding the first annular space andcommunicating therewith, cooling and expanding the vapors from saidsecond annular space into a relatively cooler condensation zone, wherebycooling, and condensation of at least one of the more volatile metalstakes place.

3. A method for separating magnesium and cadmium from mixturesconsisting essentially of these two metals by selective condensationwhich comprises; placing said mixture into an evaporating zone, heatingsaid mixture to a temperature at which melting takes place and at whichthe magnesium and cadmium have vapor pressures sufiicient to causeevaporation at a relatively rapid rate, passing the evolved vapors to acondensation zone, said vapors tracing a path having an initial,intermediate and final section through said condensation zone, saidevaporating and condensing zones being under reduced pressure, each ofsaid sections having an entrance and rearward portion, cooling theentrance of the initial section sulficiently to condense at least somemagnesium, said condensed magnesium being relatively pure, passing thevapor with only slight cooling through the initial section, impingingthe vapors on a surface cooled just below the condensing point ofmagnesium, whereby additional magnesium present in the vapors iscondensed, together with some cadmium, increasing the temperaturegradient along the vapor path of said intermediate section sufiicientlyfor heating the vapors to prevent condensation of cadmium, passing theuncondensed vapors through the intermediate section, passing the metalvapors consisting essentially of cadmium into the final section, andcooling the final section sufficiently to condense said cadmium vapors,at the rearward portion thereof.

4. The method of claim 2, where the mixture is an alloy consistingessentially of a magnesium and cadmium.

S. The method of claim 2, wherein the mixture is a. binary alloyofmagnesium and zinc.

6. The method of claim 2,wherein the mixture is a binary alloy of zincand cadmium.

7. The method of claim 4, wherein the mixture is an alloy having thecomposition Mg, 75.4%, Cd, 19.1%, Al, 4.31%, Zn, .4S%, other .74%.

8. A method of separating magnesium from cadmium in an alloy containingMg, 75.4%; Cd, 19.1%; Al, 4.31%; Zn, .45%, which comprises placing saidalloy in an evaporation zone, heating said alloy to a temperature ofabout 550 C. under a pressure of about 50 microns of mercury, passingthe evolved vapors upwardly from said evaporation zone, cooling thevapors sufliciently to condense a portion of the magnesium in relativelypure form, passing the remaining vapors through a first restrictedelongated zone, impinging said vapors at the end of said restricted zoneonto a surface cooled to just below the condensing point of magnesium,whereby additional magnesium together with some cadmium condenses,passing the remaining vapors through a first annular space surroundingsaid elongated zone and in heat exchange relationship therewith, wherebythe vapors in the first annular space are heated, passing the vaporsfrom said first annular space upwardly into a second annular spacesurrounding the first annular space and communicating therewith,expanding and cooling the vapors from said second annular space into arelatively cooler condensation zone, whereby cooling and condensation ofcadmium takes place.

9. An apparatus for separating the components of a mixture by selectivecondensation which comprises, an evaporator, a condenser incommunication with said evaporator, said condenser, having an upper andlower portion and an inner wall providing a condensing surface for themost volatile of the components to be separated, apertured separatingmeans dividing said evaporator from said condenser, a condensing surfacefor the least volatile of said components being provided by that part ofthe separating means inside the evaporator, bafile means within saidcondenser, said baffie means comprising a first inner conduit membergenerally parallel to the condenser and connected to said separatingmeans, said conduit member communicating with the evaporator via saidaperture, a cap member having side wall means surrounding said firstconduit member and spaced therefrom to define a first annular space,said cap member being spaced from the condensers inner surface to definea second annular space, the cap member having a closed top portion andapertured at the lower portion of the side wall means, said top portionproviding on its internal side an additional condensing surface, saidcap member extending up into the lower portion of the condenser, andcooling means adapted to cool the upper portion of the condenser.

10. An apparatus for separating the components of a mixture by selectivecondensation which comprises, an evaporator, a tubular condenser incommunication with said evaporator, said condenser having an upper andlower portion and an inner surface, apertured separating means dividingsaid evaporator from said condenser, a first condensing surface for oneof the components to be 5 separated being provided by that part of theseparating means inside the evaporator, baffle means within saidcondenser, said bafilemeans comprising a first tubular member having itsaxis along the axis of the condenser, said tubular member communicatingwith the evaporator via said aperture, a second tubular member havingupper and lower ends surrounding said first tubular member andconcentric therewith, said second member being longer and greater indiameter than said first member, and extending into the lower portion ofthe condenser, cap means closing the upper end of said second member,said cap means providing on its internalside a second condensingsurface, the space between said tubular members being designated a firstannular space, said second tubular member being References Cited in thefile of this patent UNITEDSTATES PATENTS 875,381 Rice Dec. 31, 19072,236,234 Hanak Mar. 25, 1941 2,458,253 Chisholm et a1. Jan. 4, 19492,508,234 DuiTey May 16, 1950 2,782,023 Weiss Feb. 19, 1957 2,814,561 DeWet Erasmus Nov. 26, 1957 OTHER REFERENCES Badger and McCabe: Elementsof Chemical Engineering, 2nd Ed, McGraw-Hill Book Co., New York.

1. A METHOD FOR SEPARATING A MIXTURE OF METALS HAVING DIFFERENT MELTINGPOINTS AND VOLATILITIES BY VACUUM DISTILLATION AND SELECTIVECONDENSATION WHICH COMPRISES; PLACING THE MIXTURE INTO AN EAPORATIONZONE, HEATING THE MIXTURE TO A TEMPERATURE AT WHICH AT LEAST SOME OF THEMETALS HAVE VAPOR PRESSURES SUFFICIENT TO CAUSE EVAPORATION AT ARELATIVELY RAPID RATE, PASSING THE EVOLVED VAPORS TO A CONDENSATIONZONE, SAID VAPOR TRACING A PATH HAVING AN INITIAL, INTERMEDIATE ANDFINAL SECTION, EACH OF SAID SECTIONS HAVING AN ENTRANCE AND A REARWARDPORTION, SAID EVAPORATING AND CONDENSATION ZONES BEING UNDER A REDUCEDPRESSURE, COOLING THE ENTRANCE OF THE INITIAL SECTION SUFFICIENTLY TOCONDENSE AT LEAST ONE OF THE LESS VOLATILE METALS, PASSING THE VAPORSTHROUGH THE INITIAL SECTION OF THE VAPOR PATH WITH ONLY SLIGHT COOLING,IMPINGING SAID VAPORS AT THE EXIT OF SAID INITIAL ZONE ONTO A SURFACECOOLED TO JUST BELOW THE CONDENSING POINT OF SAID LESS VOLATILE METAL,PASSING THE UNCONDENSED VAPORS THROUGH THE INTERMEDIATE SECTION,INCREASING THE TEMPERATURE GRADIENT ALONG THE VAPOR PATH OF THEINTERMEDIATE SECTION SUFFICIENTLY FOR HEATING THE VAPORS TO PREVENT THECONDENSATION OF THE MORE VOLATILE METALS, PASSING THE VAPORS THROUGHTTHE FINAL PATH SECTION, AND DECREASING THE TEMPERATURE GRADIENT ALONGTHE VAPOR PATH OF SAID FINAL SECTION FOR COOLING THE VAPORS WHEREBY ATLEAST ONE OF THE MORE VOLATILE SUBSTANCES IS CONDENSED.